The electrode pad 4 is stacked on the surface of the ceramic board 2 so as to overlap with the conductor 3 exposed on the surface of the ceramic board 2. The electrode pad 4 is formed to be larger in area than a cross section of the conductor 3 at the surface of the ceramic board 2, in view of positional displacement in a surface direction due to the expansion and contraction of the ceramic board 2. The electrode pad 4 has an average thickness approximately between 1 μm and 20 μm.

The electrode pad 4 includes the through holes 4A extending through the electrode pad 4 in its thickness direction, such that the through holes 4A do not overlap with the conductor 3 in a thickness direction of the ceramic board 2: namely, in the thickness direction of the insulator layers 21, 22, 23, and 24. In other words, each of the through holes 4A is positioned to avoid overlapping with the conductor 3 in the thickness direction of the ceramic board 2. In addition, each of the through holes 4A extends to reach the surface of the ceramic board 2. Accordingly, the surface of the ceramic board 2 is exposed to outside at some portions that overlap with the through holes 4A and are not covered by the electrode pad 4.

In case that the void V is formed to overlap with one of the through holes 4A in the thickness direction of the insulator layers, the void V is allowed to communicate with outside of the electrode pad 4 via the one of the through holes 4A, and discharge gas produced in the void V to the outside of the electrode pad 4 via the one of the through holes 4A.